44 ASTRONOMY • SEPTEMBER 2019
IN 1908, WHEN PERCIVAL LOWELL PUBLISHED Mars as the
Abode of Life, he saw the Red Planet as surely teeming with life. Today,
we know that’s not the case — but the search for life in the solar system
continues, and with far more tantalizing targets.
We have since learned a great deal about the icy ocean moons of the
outer solar system. We’ve seen cryovolcanoes erupting from Enceladus
and witnessed nitrogen glaciers sliding across the face of Pluto.
SATURN’S
BIG
MOON
Titan’s soupy skies drizzle complex
hydrocarbons onto the moon’s surface,
potentially providing the building
blocks of life. BY MICHAEL CARROLL
But still we wonder: What life might be
out there? If biomes have secured a foot-
hold on the distant worlds of our solar
system, what form do they take?
These days, astrobiologists are turn-
ing their sights to another alien world,
nearly the size of Mars and far more dis-
tant: Saturn’s largest moon, Titan.A different kind of life
At first blush, Titan seems an inhospi-
table place for an active biosphere. Its
opaque nitrogen-methane cocoon is the
second-densest atmosphere among all
the solid bodies of the solar system, after
Venus. Its atmospheric blanket sustains
surface temperatures of –290 degreesFahrenheit (–178 degrees Celsius). Still,
these chilly temperatures are much
warmer than the moon’s smaller sibling
Enceladus, whose daytime temperatures
hover nearly 80 F (27 C) below Titan’s.
Yet despite such frigid temperatures,
Titan has other features that might make
it more conducive to life.
Titan experiences rainfall. But instead
of water, Titan’s clouds dump liquid
methane, which is chemically similar to
the natural gas that many of us heat our
homes with. No matter what the rain is
made of, the great moon is the only world
besides Earth confirmed to have an active
rain cycle fed by evaporation from sur-
face lakes and rivers. Its river valleysSearching for life on
Titan experiences
rainfall. But
instead of water,
Titan’s clouds
dump liquid
methane from
above.
DESPITE ITS FRIGID temperature, Titan
is a tantalizing target for further exploration
because it hosts a dense atmosphere (blue haze)
and maintains an active rain cycle. NASA/JPL-CALTECH/
SPACE SCIENCE INSTITUTE